A. Technical Field
The present invention relates generally to optical communication network systems, and more particularly, to optical duobinary transmitters and receivers configured for fiber-optic communication systems.
B. Background of the Invention
The importance of high-speed optical networks is understood by one of skill in the art. High-speed networking devices typically operate at the core of a network and allow a network provider to transport large amounts of data. In order to meet the widely differing bandwidth demands of various communication applications, several communication technologies are being used, many having unique characteristics and advantages.
Wavelength Division Multiplexed (hereinafter, “WDM”) transport networks allow multiple, information-bearing wavelengths to be communicated within a single fiber. The multiplexing of optical wavelengths within a single WDM signal allows a service provider to vary the amount of bandwidth within the WDM network system by changing the number of wavelengths that are active within the system without having to drastically modify the physical infrastructure within the network.
Recent developments in photonic integration have significantly reduced the cost per end-to-end transported information bit within WDM network systems. For example, recent advances in InP photonic integrated circuit technology have enabled multi-channel WDM transmitters or receivers to be integrated on a single InP chip, reducing cost, as well as size, weight and power of optical line cards.
Advanced modulation formats are able to increase the spectral efficiency of high capacity optical networks. The choice of a particular modulation and/or detection technique depends on various engineering tradeoffs, such as tolerance to amplified spontaneous emission noise, nonlinear fiber propagation characteristics, and resilience to narrowband optical filtering due to multiple passes through optical add-drop multiplexers.
As data rates in optical communication systems have traditionally been limited by the speed of available optoelectronic components, it may be important to consider practical aspects of modulation and detection hardware when designing optical modulation formats. Finding a cost effective modulation technique for a particular system application involves aspects of modulation format and modulator technology. Three basic modulator technologies are widely in use today: directly modulated lasers, electroabsorption modulators, and Mach-Zehnder modulators.
Within the class of modulation formats with more than two symbols in the symbol alphabet, correlative coding and pseudo-multilevel modulation have received great interest in optical communications. Pseudo-multileveled data modulation formats use more than two symbols to represent a single bit and the assignment of redundant symbols to transmitted bits is data-independent. Correlative coding refers to the assignment of symbols within a signal being dependant on the transmitted data information.
Optical duobinary belongs to the general class of correlative coding formats. Correlative coding formats employ the signaling set {0, ±|E|} to take advantage of the power-detecting property of direct detection optical receivers, which automatically convert the three optical symbols to the two electrical symbols (0, |E|2}.
The duobinary signal is the fundamental correlative coding in partial response signaling. The signal is produced in the electrical domain by adding polar binary data delayed by 1 bit period to the present data. Thus, the two level polar binary data with symbols (1,−1) is converted into a three level DB signal with symbols (−2, 0, +2). The resulting duobinary signal exhibits a compressed spectrum compared with binary signal.
To perform optical phase modulation, a straight-line modulator or a Mach-Zehnder modulator may be used. In using a Mach-Zehnder modulator that is symmetrically driven around zero transmission, the modulator modulates along the real axis through the origin of the complex optical field pane, which produces exact π phase jumps at the expense of residual optical intensity dips at the location of phase transitions.
An important characteristic of an optical transmitter is the amount of optical power that it transmits. Semiconductor optical amplifiers (hereinafter, “SOAs”) are one example of an amplifier that may apply a gain to a modulated optical signal to increase the output power of an optical transmitter. However, the SOAs fast gain dynamics, which leads to waveform distortion and cross-gain modulation, poses serious challenges to employing SOAs in high-capacity WDM networks.
Many novel modulation schemes have been considered to improve the performance of SOAs. Recently, significant reduction of cross-gain modulation in the SOA was successfully demonstrated for WDM transmission with return-to-zero differential phase-shift keying (hereinafter, “RZ-DPSK”). Although saturated SOAs perform well for RZ-DPSK transmitters as power boosters and limiting amplifiers, they may not be suitable as in-line amplifiers because the input optical signals to these amplifiers usually have much lower optical signal-to-noise ratio than the signals at the transmitters.
When the SOA is highly saturated, the intensity noise at the input of the SOA can be transferred to phase noise at the output. Additionally, amplified spontaneous noise generated by the SOA may be converted into phase noise on an optical signal. If data is encoded within the phase of the optical signal (e.g., a DPSK signal), this phase noise may further degrade the optical signal being transmitted from the optical transceiver. Accordingly, the physical characteristics and noise caused by the SOA may limit is ability to efficiently operate in certain high-density WDM network environments.